Wound electromagnetic transformers are well known in the art. The problems with miniaturizing them are similarly well known. To address these problems, piezoelectric transformers utilizing the piezoelectric effect of certain ceramic materials, which can be made much smaller, especially to handle relatively low voltages, have been designed and manufactured. Additionally, piezoelectric transformers are non-flammable and produce no electromagnetically induced noise.
A variety of physical configurations of piezoelectric transformers have been designed and manufactured, including rings, circles, flat slabs and the like. One of the best known of such transformers is the so-called "Rosen" type. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 to Rosen, and numerous versions and variations of this basic apparatus are well known in the art. The typical Rosen-type transformer comprises a flat slab of ceramic that is appreciably longer than it is wide and substantially wider than it is thick. Activation of the transformer is achieved by differentially poling the ceramic the ceramic slab and attaching electrical leads to the major and minor faces of the ceramic slab to obtain electrical input thereto and output therefrom. The attachment of electrical leads to the faces of such devices by soldering or otherwise has always been a problem in the manufacturing process. Since the Rosen type transformer undergoes deformation in use, the durable attachment of leads is particularly vexing.
In order to overcome many of the problems inherent with the Rosen type piezoelectric transformer, including the need for dual poling, laminated piezoelectric transformers comprising two ceramic slabs separated by a series of metallic sheets bonded to the ceramic slab have been proposed.
A device of this type is depicted in FIG. 1 wherein a first piezoelectric wafer 30 is has two substantially parallel faces 32 and 38 that are electroplated. A second piezoelectric wafer 48 has two substantially parallel, electroplated faces 46 and 50. A first, typically pre-stressed, layer 36 is positioned adjacent electroplated surface 32. An adhesive layer 34 is disposed between the first layer 36 and adjacent electroplated surface 32 of wafer 30 for purposes of bonding the two members together. The first pre-stressed layer 36 is typically a metal having a coefficient of thermal expansion/contraction greater than that of the material of ceramic wafer 30.
A second pre-stress layer 42 is positioned adjacent the other electroplated surface 38 of wafer 30. An adhesive layer 40 is disposed between the second pre-stress layer 42 and the adjacent electroplated surface 38 of ceramic wafer 30 for purposes of bonding the two members together. The second pre-stress layer 42 is typically a metal having a coefficient of thermal expansion/contraction which is greater than that of ceramic wafer 30.
Electroplating 46 of second ceramic wafer 48 is positioned adjacent second pre-stress layer 42 such that layer 42 is between ceramic wafers 30 and 48. An adhesive layer 44 is disposed between pre-stress layer 42 and electroplated surface 46for purposes of bonding the two members together. Pre-stress layer 36 typically has a coefficient of thermal expansion/contraction greater than that of ceramic wafer 48.
A third pre-stress layer 54 is positioned adjacent the other electroplated surface 50 of ceramic wafer 48. Adhesive layer 52 is disposed between the third pre-stress layer 54 and the adjacent electroplated surface 50 of ceramic wafer 48 for purposes of boding the two members together. The third pre-stress layer 54 is typically a metal having a coefficient of thermal expansion/contraction which is greater than that of ceramic wafer 48.
After fabrication of the transformer device, ceramic wafers 30 and 48 are poled in one direction, such that when a voltage is applied across electrodes 46 and 50 or 32 and 38, the wafer will strain longitudinally. Conversely, the application of longitudinal strain to poled ceramic wafers 30 and 48 results in the generation of voltage between corresponding electrodes 46 and 50 or 32 and 38.
When a primary, or "input", voltage is applied across electrodes 32 and 38, poled ceramic wafer 30 piezoelectrically generates an extensional stress commensurate with the magnitude of the input voltage Vl. The extensional stress generated by input voltage Vl causes ceramic wafer 30 to be strained as indicated by arrow 64, which, in turn, causes ceramic wafer 48 to strain , as indicated by arrow 65, which, in turn, piezoelectrically generates a voltage V2 across electroplated surfaces 46 and 50.
At resonant frequency, the occurrence of this strain, as is clear to the skilled artisan, causes a significant deformation in the composite structure, as the voltage cycles from positive to negative. This deformation, even at a normal 60 cycles, results in significant strain and vibrational energy which is, in turn, translates into a strain on and subsequent fatigue degradation of the joints 51, 53, and 55 where the electrical leads 56, 58 and 60 are attached by soldering or otherwise. In fact, it has proven very difficult to design and implement a reliable and durable lead attachment system for such multi-layer resonating piezoelectric transformers that can withstand such strain over a long period of time.
Furthermore, because of the design of this type of very efficient transformer, as with the Rosen type transformer, it is difficult to incorporate the transformer into a printed circuit board with any degree of reliability.